The present invention relates generally to medical imaging systems and, more particularly, to determination of B1 fields associated with magnetic resonance imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
There are a variety of techniques used to determine if the B1 field produced by a magnetic resonance coil or array is homogeneous or to what degree the field is inhomogeneous. Such techniques are often referred to as B1 mapping. In general, B1 mapping techniques may either implement spatially or non-spatially resolved B1 measurements. B1 measurements are spatially resolved if one or more spatial encoding gradients are applied during acquisition and, in contrast, B1 measurements are non-spatially resolved when spatial encoding gradients are not utilized during B1 measurements. Among other things, B1 maps can be used to adjust transmit gain to produce a radio frequency (RF) pulse at a specific flip angle, to design multi-transmit channel RF pulses, and to aide in the implementation of chemical shift imaging. B1 mapping can also serve as an aide in T1 mapping and/or other quantitative MR imaging techniques. Some B1 mapping techniques are T1 dependent. That is, the signal utilized for B1 is often weighted as a function of T1 relaxation. Other B1 mapping techniques are B0 or chemical shift dependent. Still other techniques are inaccurate over certain ranges of B1 field, and/or are dependent on large RF power depositions.
Of the B1 mapping techniques, a sub-class of such techniques contains techniques that may be referred to as phase-based B1 mapping techniques. One such phase-based B1 mapping technique uses the phase accrued from a 2α-α flip angle sequence to determine B1. Although such a technique is more accurate than others over a larger range of flip angles, such a technique is B0 dependent and often relies on a relatively long repetition time (TR) requirement.
Another phase-based B1 mapping technique utilizes a B1-dependent phase produced by adiabatic hyperbolic secant half- and full-passage pulses. However, the specific absorption rate (SAR) associated with such techniques can limit the clinical application of such techniques at a high magnetic field.
It would therefore be desirable to have a system and apparatus that efficiently determines a B1 or RF field of a magnetic resonance system without some or all the aforementioned drawbacks.